Where \(T_w\)  is the transformed image of \(I^{\prime }_G\)  using structuring element \(b\) , and \(\circ \)  denote the morphological opening operator. In image processing, the top-hat transformation is a morphological operation which highlights the object smaller than the structuring element [1]}. The diameter of the structuring element is chosen 11 pixels wide as the maximum width of the blood vessel is less than 11 pixels. For the DRIVE database, the width of the blood vessel varies in the range of 1-10 pixels [2]}. The diameter of the structuring element may vary for the different database having different image resolution.
